A control device of this type is known from European Patent Publication EP 0 141 626 B1. With this control device, a defined electrical supply current, which is controlled as a function of the road speed, is provided to a solenoid of a proportional valve for changing the steering characteristics. In order to keep the power losses in the control device as low as possible, use is made of pulse width modulation, whose cycles are controlled by a generator. By means of their vibration effects, the voltage pulses generated in the process simultaneously can minimize the magnetic and mechanical friction losses in the magnetic circuit, if the cycle frequency has been selected to be correspondingly low. This means that the frictional losses in the magnetic cycle are very low when the cycle frequency is as low as possible. Therefore the selected frequency always must be a compromise and cannot offer an optimum solution for a requirement for less friction and low noise. In addition, delays occur in the control circuit between the detection of the road speed and the reaction which the driver notices at the steering wheel, which provide an inharmonious driving sense.